This invention relates to methods and apparatus for well logging, and more particularly relates to improved methods and apparatus for transmitting command and measurement signals to and from a logging sonde.
It is well known that oil and gas are found in subsurface earth formations, and that wells are drilled into these formations to recover such substances. What is not generally known is that, for various reasons, the contents of most such formations do not spontaneously discharge into the well bore upon being penetrated. Furthermore, it is usually necessary to survey or "log" substantially the entire length of the borehole to locate those formations of interest before a well can be completed to produce oil or gas.
There is no single well logging technique or device which can provide a direct indication of oil or gas in a particular formation of interest. Instead, it is common practice to measure the lithology of the earth substances adjacent to the borehole, whereby those formations of potential interest may be identified. Since no one lithological parameter or characteristic, or even any single combination of such parameters, can of itself provide a definitive and conclusive indication of the presence of oil and gas in commercial quantities, there has been a continuing need to make as many different logging measurements as possible.
The expense involved in logging an oil or gas well is quite high, however, and is usually dependent on the time required to traverse the borehole with the sonde or logging tool. Also, well logging measurements made during one passage of the sonde may not directly correlate to measurements made during another passage of the sonde because corresponding increments of such measurements will not necessarily have been taken at the same borehole depth. For these and other reasons there has been an increasing need to provide methods and apparatus for making a plurality of different but correlated measurements during the same passage of the sonde through the borehole.
One such plurality of measurements relates to the traversal time of acoustic energy across an increment of the formation, which may provide indications of formation porosity and the like, as well as a measurement of the relative amplitudes of such energy along the borehole. The latter measurement is frequently made, for example, during a logging operation known in the art as a cement-bond log.
In a typical well completion operation, a well casing is placed within the borehole and cement is pumped into the annulus for purposes of isolating production zones, preventing migration of undesirable fluids, and the like. It is frequently desirable to obtain measurement of the degree and quality of the bonding of the cement to the casing as an indication of this sealing effect, as it is found that at times the cement may not entirely fill the annulus at certain increments along the borehole, or that a channelling effect may occur.
In other instances, it may be desirable to know the extent of packing of the earth formation itself about a drill pipe which, for example, may be stuck in the borehole. Often the point of compaction may behave in a manner similiar to the aforementioned cement-bonded pipe in severely attenuating an acoustic signal, such that the same amplitude measurement may be employed in both the cement bonding and stuck pipe or formation packing situations.
One problem associated with employment of conventional logging tools for purposes of making the aforementioned acoustic travel time measurements or seeking to recover the entire acoustic signature traversing a portion of the formation on the one hand, and attempting to measure the amplitudes of such acoustic signals for the cement bond log or related purposes just described, relates to the relative magnitude of the signals involved in both situations. It will be readily apparent that, due to the aformentioned severe attenuation of acoustic signals along the casing in instances of formation packing and cement bonding, substantial signal amplification may be required. However, in instances when the acoustic signature traversing the formation is sought to be recovered, if the same signal amplification is employed, it will be readily apparent that the signal amplification means may be driven into saturation severely distorting the resulting measured acoustic signature. On the other hand, because the signal amplitude provides information as to the quality of cementation, decreasing as the quality increases to a very greatly attenuated extent, it will be appreciated that if the appropriate amplifier gains or attenuation are adjusted so as to be compatible with acoustic travel time or acoustic signature measurements in which the signature traverses the formation, there may not be sufficient amplification to recapture the amplitude information of the greatly decreased signal in the case of the cement bond log for example.
From the foregoing, it will be readily apparent that it was highly desirable to provide means for selectively adjusting the amplitude of the measured acoustic signature dependent upon the particular type of logging operation being conducted. Thus, one solution in the prior art was to provide such an adjustable acoustic signal attenuation or amplification means, whereby the attenuator was set to one preselected gain and an entire logging operation was conducted for cement bond log purposes, for example, and the logging tool was then adjusted to a second preselected gain or attenuation for purposes of conducting the conventional log of acoustic energy traversing the formation. It should be obvious, however, that this attempted solution suffered from the drawback of requiring two entirely separate traversals of the sonde through the borehole, which, as previously pointed out, was a serious drawback.
In recognition of this problem, the prior art thereafter provided means for selectively adjusting the amplification or attenuation from the surface while the logging sonde was disposed in the borehole. However, this solution employed the use of separate logging cable conductors for controlling amplitudes on the one hand, and still other conductors for purposes of determining the firing order of the various transmitters and receivers in the acoustic logging tool. Also, as previously noted, this attempted solution had serious drawbacks due to the required committment of additional logging conductors already in short supply for performing both of these functions. Moreover, no means was provided for automatically cycling the gain or attenuation between the two preselected levels during the logging operation such that, even if it was possible to obtain an acoustic log during one borehole traversal at the two preselected gains, the logging operation was thus so inefficient as to be commerically impractical.
These and other disadvantages are overcome by the present invention, however, wherein improved methods and apparatus are provided for deriving a plurality of different logging measurements with a reduced interference between the various command and logging signals during the same trip through the borehole.